A second-generation mobile communication technology (2nd Generation, 2G) or a 3rd-generation mobile communication technology (3rd Generation, 3G) network has created an overall coverage situation after being deployed for many years. Meanwhile, with the development of network technologies, some PS domain networks providing only a packet switched (PS) domain, such as a long term evolution (LTE) network, in an early stage of network construction, has gradually covered some city areas and telephone traffic hotspot areas. Therefore, in current communication networks, the PS domain networks coexist with 2G/3G networks.
Corresponding to different communication networks, different bearing mechanisms are available for voice services. Specifically, in a 2G/3G network, a voice call supports a session service borne by a circuit switched (CS) domain, while in a PS domain network, a voice call supports a session service borne by a PS, which is also referred to as a voice over Internet Protocol (VoIP) service, for which an IP multimedia subsystem (IMS) ensures the quality of service.
A technical solution is desired for enabling an MSC server and an SGSN to interact with the same MME, thereby switching a voice service from a CS domain of a global system for mobile communications (Global System of Mobile Communication, GSM)/universal mobile telecommunications (Universal Mobile Telecommunications System, UMTS) network, which is specifically a GSM/UMTS network, to a PS domain of an LTE/high speed uplink packet access (High Speed Uplink Packet Access, HSPA) network.